An interinstitutional cooperative study is proposed to determine the effectiveness of acetylsalicylic acid (aspirin) in the reduction or prevention of transient cerebral ischemic attacks (TIA) and amaurosis fugax. In addition, the frequency of cerebral and myocardial infarction in long-term follow-up will be evaluated. This will be a controlled clinical trial in which only the hemispheric type of TIA or monocular blindness will be included. Primary vertebrobasilar insufficiency only or with potentially fatal complicating medical conditions, will be excluded. Care will be taken to exclude individuals with blood dyscrasia or specific idiosyncrasy to aspirin. Subjects will be allocated to subgroups on the basis of a clinical decision to operate or not to operate. In each of the two subgroups, subjects will be assigned to treatment with aspirin or a placebo on the basis of a random statistical design. This design will enable the investigators to make meaningful comparisons between therapies in each subgroup. Strict surveillance will be maintained of the patient's adherence to the instructions for therapy. The administration of the active drug and placebo will be on a coded, double blind basis. A mechanism is provided in the project design for breaking the code in the event of clinical necessity. The facilities to be used for data collection, storage, retrieval and analysis will be those currently being used for the Central Registry of the Joint Study of Extracranial Arterial Occlusion, and for patient evaluation the Bowman Gray School of Medicine and North Carolina Baptist Hospital. A total of twenty-five patients will be entered annually.